1. Field of the Invention
The present invention relates to hydraulic pumps, such as those that have pistons that move radially against an eccentric shaft, and more particularly to mechanisms for controlling the flow of fluid through the cylinders in which the pistons move.
2. Description of the Related Art
A common type of radial piston pump comprises a body with a plurality of cylinders radially disposed around a drive shaft that is rotated by an external motor or engine. A separate piston is slideably received within each cylinder, thereby defining a chamber at the interior of the cylinder. The drive shaft has an eccentric cam and the pistons are biased by springs to ride against that cam. As the cam rotates, the pistons slide reciprocally within the respective cylinders, thereby reducing and expanding the volume of the cylinder chambers in a cyclical manner. The smallest volume occurs at the top dead center point of the piston cycle and the largest volume occurs at the bottom dead center point.
An inlet port supplies fluid to an inlet passage that has a separate inlet into each cylinder. Every cylinder also has an outlet that is coupled by a separate outlet check valve to an outlet passage that leads to the outlet port of the pump. U.S. Pat. No. 3,434,428 discloses a pump of this configuration. The pump in that patent also has a throttle plate with apertures associated with the inlets for the cylinders. The throttle plate is rotated by an actuator to vary alignment of the apertures with the inlets and thereby alter the amount of fluid flowing between the common inlet passage and each cylinder inlet.
With this type of pump, as the piston moves from the top dead center point, fluid is not initially drawn into the expanding cylinder chamber because the location of the piston blocks the inlet. The piston has to move a considerable distance from the top dead center point before the inlet is unblocked and fluid from the inlet passage is drawn into the expanding cylinder chamber. After the bottom dead center point, the volume of the cylinder chamber begins reducing, however, the inlet still is open which prevents outlet check valve from opening. Here too, the piston must move some distance before the piston blocks the inlet and causes pressure in the cylinder chamber to increase. As the piston starts to pump, the sealing land of the piston is low in the cylinder and high pressure fluid leakage occurs thereby making this form of aspiration initially in-efficient. Eventually the pressure rises to a level that forces the outlet valve to open an outlet path through which the fluid is exhausted from the cylinder chamber. That exhausting continues until the piston again reaches the top dead center point.
A drawback of this type of pump is that during a dead portion of the piston cycle, between bottom dead center point and when the inlet becomes closed, no pumping action occurs. Specifically, fluid is neither being expelled from the cylinder nor being drawn into the cylinder during that dead portion, which can be a third of the piston cycle as shown in FIG. 6 of the U.S. Pat. No. 3,434,428. This inactive time and initial short sealing length results in a sizeable inefficiency. In addition this type of pump requires a relatively long piston stroke to accommodate the dead portion of the piston cycle, which increases the diameter of the pump.
These prior radial piston pumps also had a relatively large diameter due to the outlet valves and the outlet passage being located radially outward from each cylinder. For many machines, the amount of space for the pump is limited, thus it is desirable to reduce the size of the pump. More specifically, many times the pump is mounted alongside an engine or transmission and the radial space is limited preventing the installation of typical radial piston pumps.